Method of producing heat-shrinkable polyethylene terephthalate film



United States Pateflt O hilv METHOD OF PRODUCING HEAT-SHRINKABLE P OLYETHYLENE TEREPHTHALATE FILM Robert LaurencesRichards, Jr., Kenmore, N.Y., assignor .to E. I. du Pont de Nemours and Company, Wilmington, Del a corporation of Delaware No Drawing. Application December 31, 1957 a J Serial No. 706,247

Claims. (CI. 18-48) This invention relates to heat-shrinkable film of synthetic linear polyester, and more particularly to the prosynthetic linear organic polymers such as polyethylene terephthalate which are capable of being formed into translucent or transparent films having excellent physicalproperties. Certain of these properties such as. tensile strength, impact strength, flex I pendicular to the first direction 2,923,132 Patented Mar. 15, 1960 2 temperatures below about 85 C. These and other objects will more clearly appear hereinafter. 7 p

The foregoing objects are realized by the present invention which, briefly stated, comprises stretching substantially amorphous polyethylene terephthalate film first in one direction and thereafter in a second direction perto an extent such that the final film thickness is reduced to approximately the original thickness of the film (the thickness of the.

film prior to stretching), the film being stretched to a lesser amount in the first direction such that the ratio of the amount of stretch in the first direction to the amount of stretch in the second direction is equal to a value within the range of from 0.65 to shrinkage in both directions, as well as the improved physical and chemical properties inherent in oriented film.

The preferred subject for treatment in accordance with the process of this invention is film formed from polyethylene terephthalate, as described in US. Patent Number 2,465,319 to Whinfield and Dickson. Polyethylene terephthalate may be prepared by the condensation of ethylene glycol and terephthalic acid, or prefer-.

ably by carrying out an ester interchange reaction between ethylene glycol and dialkyl esterterephthalic acid, e.g., dimethyl terephthalate. Films of polyethylene terephthalate may be prepared by extruding the molten polymer through a narrow orifice and chilling the polymer in film form. I

' It is to be understood, however,

invention comprehends films of any synthetic linear terephthalic ester polymer derived by reacting a glycol ple, a polyethylene ter ephthalate film oriented by stretchwe to an X Pl...Q5f mesfi s niti di'r'eetiofi's aiid heat setat 85 C. exhibitsashrinkage of approximately .iQfhfinAhdiin: dire tion. of; s ret h and. 46%-'in "tli"second direction (mutually perpendicular to the first direction of stretch). Ibis unbalanced shrihkk...

dimensions in both age'ratio i si of great disadvantage for certain packaging uses wherein a greater degree patierns is a prerequisite. I v

By the term shrink or shrinkage as used is meant the amount the film contracts in one or both directions when immersed in boiling waterjIQOf C.) under subof balance of shrinkage stantially n6 tension. "'By the term is meant the film shrinks to the same extent in either direction within the limits of experimental error in measurement (25%). By the term heat-setting temperature is meant the ambient temperature (temperature of the atmosphere immediately, surrounding the. film).

to which the film is subjected -'while under not the actual temperature of the film itself.

A principal object of' thisjinvention, therefore, isv to tension and provide a heat-shrinkable, biaxially oriented polyethylene terephthalate film having substantially the same degree of shrinkage'in both directions of the film. further object is to provide; a proe'ess for producing a heatshrinkable, biaxially oriented polyethylene terephthalate film 'having substantially-balanced shrinkage characteristics; Still another-object is .to provide a heat-shrinkable, biaxially, oriented polyethylene terephthalate film halving substantially the samedegree of: shrinkage in both direct'im' and "Wil s sub tant y d o a y sta le; at

balanced shrinkage selected from the series HO(CH ),,OH where n is an integer from 210 inclusive, terephthalic acid, or esterforming derivatives thereof, or low molecular weight esters thereof, and from 020% by weight of a second acid or ester thereof; said second acid being selected from such acids as isophthalic acids, bibenzoic acid, naphthalic acid, 2,5-dimethyl terephthalic acid, and bis-p-carboxy phenoxy ethane.

As mentioned above, polyethylene terephthalate oriented by stretchingto substantially the same extent in both directions exhibits a substantially greater degree of shrinkage in the second direction of stretch. Inasmuch as film shrinkage normally increases with an increase in the degree of stretch, one skilled in the art would expect that in order to produce a balanced film with respect to shrinkage it would be necessary to stretch the film to a greater extent in the first direction than in the second direction of stretch. Unexpectedly it has been discovered that if, iiiorienting.polyethylene terephthalate and like polyester film by stretching the film in both directions such that the thickness is reduced to approximately A-V its original thickness, the film is stretched to a lesser degree in the first direction of stretch than in the second direction, an oriented polyethylene terephthalate film having substantially balanced shrinkage results. For reduction of film thickness to a value less than ,6 the original thickness of the film (the thickness of the filmprior .to stretching) but not less than A, e.g., for,

reduction to, say, or 4;, the ratio of the amount of stretch in the first direction to the amount of stretch in the second direction will be influenced by the speed at. which the film is stretched. With total reductions in thickness of 4;, however, regardless of the speed at which the film is stretched, the film will possess substantially balanced'shrinkage patterns if a stretch ratio of 0.65 to 0.75is utilized. The level of shrinkage will, of course, be controlled by the temperature at. which the film is heat-set and the speed at which the film is stretched.

. Because of the commercial importance of such films, this 0.75. The resulting biaxiallyoriented film exhibits substantially the same degree of that the present application will concern itself mainly with the produc' tion of polyethylene terephthalate films from substantially amorphous polyethylene terephthalate having approximately the originalithickness of the film which exhibits a substantial degree of balanced shrinkagef'in both directons of the film. i

Polyethylene terephthalate films prepared as described above (total reduction in thickness of approximately 34;) also unexpectedly, in addition to exhibiting substantial balanced shrinkage in either direction, possess enhanced physical properties comparable with those of a film stretched 3 times its original dimensions in both directions.

The optimum temperature at. which the film is stretched will generally fall within the range of from 70105 C., and, in the preferred embodiment of the invention, the film after stretching is heat-set at a heats e'tting temperature of from 85-100 C. while main taining the film under suflicient tension (during the heatset interval and subsequent cooling), to prevent shrinkage during the heat-setting operation. This latter step insures a film having dimensional stability below the heat-shrinking temperature defined hereinabove. As stated previously, the total stretch imposed on the film will be such asto reduce the film to a final thickness of approximately ,6 the thickness of the film prior to stretching in the first direction. Preferably, the film is stretched to about 2.5X, where X signifies the original dimension of-the film, in the first direction and to an eiitentf ofabout' 3.7X in the second direction, at temperatlu resfwithin the range of 70-105 C., and heat-set \iiider restraint at 85 -100 C. i

The film may be stretched by anyconventional stretchs u r V ingfm eanjs; lknown tone, art. It may be. stretched to a V 4 1 2.6 times the original length of the film in the lqngit nd i; nal direction and than was" eit'ent of 'itlftimes the original width in the transverse direction (MD to TD ratio of 0.74) at temperatures of 8085 C. The film was heat-set under restraint at 85 C. The shrinkage of the film was measured'as follows:

The stretched, heat-set film was out into test samples measuring 5 x 5", two edges being parallel in the longitudinal direction and two being parallel in the transverse, direction. The samples were grasped-near the center by a pair. of forceps and immersed in a beaker of boiling water (100 C.). After 30 seconds the samples were removed and mopped dry with a paper, towel. The dimensions of thesquare, sample were measured to determine the amount of shrinkage in each direction. Three samples were run, and an average of the shrinkage of the three samples was taken as the recorded shrinkage of the film.

In this case, the average shrinkage was 27%. in each direction EXAMPLE 2 In a similar manner to that described in Example 1, substantially amorphous polyethylene terephthalate film was? stretched in both directions at temperatures of 30290? C. The stretch ratios (MD/TD) and the film speeds were varied; as hereinafter indicated, the total" stretch in each instance beingsuch that the thickness of; the filrn was reduced to of its. original value; film was heatset under" restraint at the indicated tempefatures. The film was testedatulflo C. fortshrinkago in thei manner described hereinbefore. Table I, below, lists etstretch ratio and the precentshrinkage of the. film it both directions. at film stretching speeds of 20, am- :8Q yards-per minute.

From able I it can be seen that when the final film thickness is reduced to about %the original thickness, and the MD to TD stretch ratio is 0.65 to. 0.75, a film Table I- EFFEO'I. DESTRETOH RATIO ON FILM-BH-RINKAGE TOTAL REDUCTION IN FILM THICK- NESS== 9X TEMPERATURE=H5 0.

Percent Shrinkage at Speeds o t,- StretchRatlo I n h I 20Yds.pe r MtnJ fiQYds. per Min. 7 80 YjdsperzM tnk MD 'ID MD/TI) MD To run/TD MD Urn MD/TD Ml) 'rn MD/TD 2.5 5.7 o.es 26 2a 1.12 31 35 0.9 it 31 1.10 3.0 3.0 1.0 so 40 0. as as 49 0.68 as as 0.68 3.5 2.6, 1. as 31 61 0.51 as 02 0. as, 31 to 0.52 to 2. 3 ma 32 59 0.55

1 Film heat-set at temperature 0185 O. for 20 seconds; Film heat-set at temperature of 85 0. for 8 seconds; a Film heat-set at temperature on-35 C. for 5 seconds th etp nciples and practiceof this tinventiona.

EXAMPLE 1 Snbstantially amorphous as-cast polyethylene. tereph thalate film was stretched in both directions in a stretch ingapparatus more fully described in Scarlett UJS. ap'- plication serial-Ne; 2875354'-'(filed May 112; 1952 new PatenLNo; 2,823,421; at a filir'r speed of 2p yards pe minute. The film was stretched first-to the extent 0fis produced haying. substantially. balanced shrinkage in both directions irrespective; of film speed The level of; shrihkage is deterrnineclbytwo factors: the: film speed: and ,theptemperature, at which the film' is heat-set.

Table; II below; listingthe. heat-setting. temperature,

thevdensity; percenticrystallinity ofl the film andpercent.

shrinkage, in both the'lon'gitudinal transverse, direc "tiqnslat film speeds bill); 'S QYa'nd' yards perminute illustrates the effector" heatsetting rtemperaturei and film tions tofafiqtal, reduction; in thickn'ees @136 the, original; thickness, wherein ,the MDt 'rnjta is falling vt ithin tlie defined li'mitsbf "OLGS jto 0.15 has b een'utiliz ed? (i tliiscase .5 2.5x stretch 113. the longi'tndinal; direeti en and-a 3%7-X- -st'retch i'rftlie transvrse dirctionl EFFECT OF CRYSTALLINIIY ON FILM SHRINKAGE OF A FILM STRETCHED 2.5X MD BY 3.7X 'ID, AT TEMPERATURES OF 80-85" C. AND HEAT-SET FOR ONE MINUTE AT TABULATED TEMPERATURES Film Speed 20 yards per Minute 50 Yards per Minute 80 Yards per Minute Temperature, C.

Percent Shrinkage Percent Shrinkage Percent Shrinkage Density Crystal- Density Crystal- Density Crystallmlty linity linity 7 MD TD MD '1D MD TD From Table II it can be seen that as the film speed is increased, the level of shrinkage increases. Conversely, the higher the heat-setting temperature and the higher the percent of crystallinity of the film, the lower the level of shrinkage. The balance obtained by stretching at the above-mentioned MD to TD stretch ratios, however, is substantially unaffected by changes in heatsetting temperature and film speed.

The present invention not only presents a highly successful method for the production of a heat-shrinkable polyethylene terephthalate film which exhibits a substantially balanced two-directional shrinkage, but also eifectively provides a method for controlling the degree of shrinkage at any desired level. Such a film has a great potential market both in the food packaging field and other types of industrial packaging. A film with balanced shrinkage is particularly useful in applications where skin-fitting is necessary. Films which do not shrink to substantially the same degree in both directions present a heat-sealing problem in that they are more difiicult to seal, and they are also less adaptable to formation of bags in standard bag machines.

I claim:

1. A process for forming biaxially oriented, heatshrinkable polyethylene terephthalate film exhibiting substantially the same degree of shrinkage in both directions of the film which comprises stretching substantially amorphous polyethylene terephthalate film first in one direction and thereafter in a second direction perpendicular to the first direction to an extent such that the final film thickness is approximately one-ninth of the thickness of the film prior to stretching, the film being stretched to a lesser amount in the first direction such that the ratio of the amount of stretch in the first direction to the amount of stretch in the second direction is equal to a value within the range of from 0.65 to 0.75.

2. A process for forming biaxially oriented, heatshrinkable polyethylene terephthalate film exhibiting substantially the same degree of shrinkage in both directions of the film which comprises stretching substantially amorphous polyethylene terephthalate film at a temperature within the range of from 70105 C., first in one direction and thereafter in a second direction perpendicular to the first direction to an extent such that the final film thickness is approximately one-ninth of thestantially the same degree of shrinkage in both directions of the film which comprises stretching substantially amorphous polyethylene terephthalate film first in one direction and thereafter in a second direction perpendicular to the first direction to an extent such that the final film thickness is approximately one-ninth of the thickness of the film prior to stretching, the film being stretched to a lesser amount in the first direction such that the ratio of the amount of stretch in the first direction to the amount of stretch in the second direction is equal to a-value within the range of from 0.65 to 0.75, and thereafter heat-setting the resulting stretched film at a temperature of from -100 C. while maintaining the film under tension.

4. A process for forming biaxially oriented, heatshrinkable polyethylene terephthalate film exhibiting substantially the same degree of shrinkage in both directions of the film which comprises stretching substantially amorphous polyethylene terephthalate film at a temperature within the range of from 70-105 C., first in one direction and thereafter in a second direction perpendicular to the first direction to an extent such that the final film thickness is approximately one-ninth of the thickness of the film prior to stretching, the film being stretched to about 2.5X, where X is the original dimension of the film, in the first direction, and to about 3.7X inthe second direction.

5. A process for forming biaxially oriented, heatshrinkable polyethylene terephthalate film exhibiting substantially the same degree of shrinkage in both directions of the film which comprises stretching substantially amorphous polyethylene terephthalate film at a temperature within the range of from 70105 C., first in one direction and thereafter in a second direction perpendicular to the first direction to an extent such that the final film thickness is approximately one-ninth of the thickness of the film prior to stretching, the film being stretched to about 2.5X, where X is the original dimension of the film, in the first direction, and to about 3.7X in the second direction, and thereafter heat-setting the resulting stretched film at a temperature of from 85- C., while maintaining the film under tension.

' References Cited in the file of this patent UNITED STATES PATENTS 2,465,150 Dickson Mar. 22, 1949 2,597,643 Izard et al May 20, 1952 2,627,088 Alles et al Feb. 3, 1953 2,784,456 Grabenstein Mar. 12, 1957 2,804,652 Balkan Sept. 3, 1957 2,812,550 Chavannes Nov. 12, 1957 2,823,421 Scarlett Feb. 18, 1958 

1. A PROCESS FOR FORMING BIAXIALLY ORIENTED, HEATSHRINKABLE POLYETHYLENE TEREPHTHALATE FILM EXHIBITING SUBSTANTIALLY THE SAME DEGREE OF SHRINKAGE IN BOTH DIRECTIONS OF THE FILM WHICH COMPRISES STRETCHING SUBSTANTIALLY AMORPHOUS POLYETHYLENE TERETHALATE FILM FIRST IN ONE DIRECTION AND THEREAFTER IN A SECOND DIRECTION PERPENDICULAR TO THE FIRST DIRECTION TO AN EXTENT SUCH THAT THE FINAL FILM THICKNESS IS APPROXIMATELY ONE-NINTH OF THE THICKNESS OF THE FILM PRIOR TO STRETCHING, THE FILM BEING STRETCHED TO A LESSER AMOUNT IN THE FIRST DIRECTION SUCH THAT THE RATIO OF THE AMOUNT OF STRETCH IN THE FIRST DIRECTION TO THE AMOUNT OF STRETCH IN THE SECOND DIRECTION IS EQUAL TO A VALUE WITHIN THE RANGE OF FROM 0.65 TO 0.75. 